Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
A common type of automatic transmission utilizes a collection of shift elements. Various subsets of the shift elements are engaged to establish the various speed ratios. A common type of shift element utilizes a clutch pack having separator plates splined to a housing and interleaved with friction plates splined to a rotating shell. When the separator plates and the friction plates are forced together, torque may be transmitted between the housing and the shell. A controller controls the torque capacity on such a friction clutch by adjusting the force squeezing the clutch pack. Friction clutches are capable of transmitting torque in the presence of relative speed between the separator plates and the friction plates. Some drag torque is transmitted even when a friction clutch is disengaged reducing efficiency.
Another type of shift element is a one-way-clutch. A one-way-clutch permits relative rotation between two races in one direction, but prevents relative rotation in the opposite direction. One-way-clutches have advantages and disadvantages relative to friction clutches. Shift quality is improved when the off-going element is a one-way-clutch because the clutch passively releases at the correct time. Generally, the parasitic drag of one-way-clutches is lower than that of friction clutches. However, one-way-clutches cannot transmit torque in the presence of relative rotation between the races. Some one-way-clutches are selectable. A selectable one-way-clutch (SOWC) can be actively controlled to be in one of multiple states which vary in terms of which directions of relative rotation are permitted. For example, a selectable one-way-clutch may permit relative rotation in one direction but not the other in one state and preclude relative rotation in both directions in another state. Like non-selectable one-way-clutches, selectable one-way-clutches are not capable of transmitting torque in the presence of relative rotation.